A conventional "vapor-compression" refrigeration cycle employs a single refrigerant that is circulated through a conduit between a heat sink and a thermal load. This cycle relies on the thermodynamic principles of adiabatic compression (temperature increase), isenthalpic expansion (temperature decrease) and latent heat of vaporization or condensation of a fluid.
Refrigerants, such as chlorofluorocarbons, hydrochlorofluorocarbons and hydrofluorocarbons, are typically liquids at ambient temperatures. At one stage in the refrigeration cycle, the refrigerant passes through a compressor that increases its pressure and temperature, causing it to release heat as it condenses from a vapor to a liquid form in a condensing heat exchanger. At another stage in the cycle, the liquid refrigerant passes through an expansion valve to reduce its pressure and temperature, creating a two phase fluid. This reduction in temperature causes the refrigerant to absorb heat and evaporate within the evaporative heat exchanger. In this conventional cycle, the "working fluid", which is compressed and expanded as it circulates, and the "heat transfer fluid", which accepts heat from the thermal load and rejects heat to the heat sink, are the same thing, namely the volatile refrigerant. The compressor and expansion valve are physically separated, the compressor being at the "hot end" of the cycle and the expansion valve being at the "cold end" of the cycle. The condensing heat exchanger rejects heat to the heat sink while the evaporative heat exchanger absorbs heat from the thermal load.
Regenerative thermodynamic cycles that use regenerators for periodic heat exchange are known in the prior art. In most cases the regenerator is a material which has a large thermal mass and heat transfer surface. In typical regenerative cycles the regenerator is a passive element that is not capable of doing work and whose purpose is to transfer heat back and forth to a working gas periodically during the cycle to enable larger temperature spans to be achieved. The working gas continues to be compressed at the hot end of the cycle and expanded at the cold end of the cycle. Moreover, the working gas is the same gas which is used to transfer heat from the cooled space to the environment via heat exchangers. Stirling, Gifford-McMahon and Orifice Pulse Tube devices are all examples of prior art refrigeration systems employing passive regeneration.
Stirling cycle devices operate on a regenerative thermodynamic cycle, with cyclic isothermal compression and isothermal expansion of the working fluid at different temperature levels, separated by constant volume flow through regenerators with a temperature span from the two different temperatures of compression and expansion. Stirling cycle devices have been used as heat engines, heat pumps, and refrigerators.
In a Stirling cycle machine operating as a prime mover, the working fluid isothermal compression takes place in the hotter chamber, while most of the isothermal expansion takes place in the colder chamber. Some of the heat introduced at the hot chamber is converted to work in the prime mover and the residual heat is rejected at the cold chamber. As will be appreciated by those skilled in the art, when the Stirling cycle is used in a refrigerating machine rather than a prime mover, the working fluid isothermal expansion that absorbs heat occurs in the cold chamber while the isothermal compression of the working fluid, during which heat is rejected, takes place in the hot chamber. In either type of machine the working fluid is shifted between the two chambers through a passive regenerator which is not itself capable of doing work.
In prior art Stirling cycle machines, the "working fluid" which is alternatively compressed and expanded may either be a gas or liquid. For example, U.S. Pat. No. 5,172,554 dated Dec. 22, 1992, Swift et al., discloses a Stirling thermodynamic cycle refrigerator that utilizes a single phase solution of liquid .sup.3 He as the working fluid. The liquid .sup.3 He may be present in superfluid .sup.4 He. As in conventional Stirling cycles, a passive regenerator is employed as a thermal reservoir that maintains a temperature difference between the compressor and expander and functions as a thermal reservoir that cyclically exchanges heat with the working fluid. Work is applied to the working fluid during the Stirling cycle in the compressor and expander rather than within the passive regenerator itself.
U.S. Pat. No. 4,353,218 dated Oct. 12, 1982, Wheatley et al., relates to a heat pump/refrigerator using working fluid that is continuously in a liquid state. The Wheatley apparatus includes a pair of heat exchangers respectively coupled to a thermal load and a heat sink, a displacer forming a pair of reservoirs coupled to the different heat exchangers, a regenerator connecting the heat exchangers, and means for compressing a working fluid that can pass between the reservoirs by way of the regenerator and a heat exchanger. The working fluid may consist of, for example, compressed polypropylene. As in other similar prior art systems, the regenerator is utilized to transfer heat from the working fluid leaving one heat exchanger into fluid leaving the other heat exchanger and does not input work into or remove work from the system.
"Active regenerators" utilize heat transfer materials that not only have large thermal masses and heat transfer surfaces but are also capable of doing work during a thermodynamic cycle. Heretofore active refrigerants have been solids, such as magnetic materials or elastomers. For example, U.S. Pat. No. 4,704,871, Barclay et al., issued Nov. 10, 1987, relates to magnetic refrigerators employing paramagnetic or ferromagnetic materials. When such materials are adiabatically passed into and out of a magnetic field (such as produced by a superconducting magnet) their temperature alternatively increases and decreases. This is referred to as the magnetocaloric effect. By way of example, if Gadolinium at room temperature is adiabatically subjected to a magnetic field of about 8 Tesla it will increase its temperature by about 12-14 K. A refrigeration cycle may be enacted by passing a heat transfer fluid between hot and cold heat exchangers in a periodic flow as the magnetic material is alternatively adiabatically magnetized and demagnetized.
One significant problem associated with active regenerative systems employing the magnetocaloric effect is the cost of developing adequate adiabatic temperature changes especially for near room temperature use. Magnetic systems require powerful superconducting magnets to achieve magnetic fields large enough to cause modest temperature ratios. Such superconducting magnets are very expensive and not practical for many applications and the energy required to keep the superconducting magnets cold makes the entire cycle inefficient with the exception of very large systems.
Elastomeric materials may also be used as an active heat transfer element in a regenerative system. U.S. Pat. No. 5,339,653 dated Aug. 23, 1994, DeGregoria, describes refrigeration cycles based on the thermoelastic effect in which certain elastomers, such as rubber, warm upon stretching and cool upon contracting. In particular, a regenerative bed may be formed comprising a porous matrix of elastomeric sheets arranged in layers with spacers between the sheets defining fluid flow channels. Work may be inputted into or removed from the system by periodically stretching and contracting the elastomeric sheets to effect temperature changes. A circulator passes a heat transfer fluid through the porous matrix in one direction when the bed is at one temperature or stretch and in the reverse direction when the bed is at a different temperature or stretch.
The significant problems associated with active regenerative systems employing the thermoelastic effect include the large strains (.about.4-10) required to achieve modest temperature change (.about.20 K), hysteretic effects and crystallization of the elastomer after prolonged use or upon cooling significantly below room temperature.
While the use of solid heat transfer regenerative materials capable of doing work, such as magnetic or elastomeric materials, is known in the prior art, the use of an active or "working" fluid capable of doing work in a regenerative refrigeration cycle has not been previously described as a means of improving thermal efficiency. The need has therefore arisen for an active regenerative refrigerator that comprises a working fluid separate from the heat transfer fluid and which is distributed over the temperature profile of a regenerative bed. The need has also arisen for an active regenerative refrigerator of modular design that may be easily tailored to meet the heat transfer requirements of different applications, thereby achieving optimum versatility.
Since the present invention achieves improved thermodynamic efficiency, it has many potential cryogenic and near room temperature applications. For example, vehicles that operate on liquefied natural gas are particularly attractive as an alternative to gasoline-based vehicles in that they utilize a domestically available fuel, generate less pollution and have significantly lower maintenance costs. The refueling stations needed to service vehicles operating on liquefied natural gas will require relatively inexpensive refrigerators to liquefy the gas delivered through pipelines that operate at ambient temperature.
Numerous high temperature superconductor devices provide the promise of improved electronic performance provided cost-effective refrigeration systems are available to cool the electronics down to near or below liquid nitrogen temperatures. The present cost of cryogenic cooling systems, however, makes circuitry that utilizes superconductors impractical for consumer applications.
The generation of liquid oxygen for use in sewer treatment plants would likewise benefit from more cost-effective refrigeration systems. Oxygen is bubbled through aerobic digestion ponds to increase the speed at which waste products are oxidized. The oxygen is typically generated on site by cryogenic liquefaction of air. It would be advantageous to be able to increase the efficiency of such cryogenic systems, thereby lowering the cost of generating the liquid oxygen.
Prior art cryogenic refrigeration systems with large cooling capacities typically depend upon large compressors that generate a great deal of vibration and have limited lifetimes. The need to isolate the vibration and reduce the noise further increases the cost of the systems. It would be clearly advantageous to avoid cryogenic systems that have moving parts and seals requiring periodic replacement.
With the introduction of the Montreal Protocol the initial objectives of reducing emissions of ozone depleting gases, most of which came from the near room temperature refrigeration industry, have been stated. Its implementation has caused the substitution of the CFC refrigerants with similar compounds with less ozone damaging potential. Unfortunately some of the new ozone friendly refrigerants are inferior to previous refrigerants and have reduced the efficiency of some refrigeration equipment.
The newest environmental challenge is the reduction of greenhouse gas emissions. In the case of the near room temperature refrigeration industry, increasing the efficiency of refrigerating devices will help reduce such emissions.
There are many applications in the near room temperature market including air-conditioners, refrigerators, freezers and heat pumps. Vapor compression technology is used in the vast majority of products for these markets and has been under continuing improvement for approximately 100 years. The efficiency of the current products can be increased slightly but only with an increase in capital cost. A refrigerating system with improved efficiency and similar or reduced capital cost would be highly advantageous.